My question is, how much really things change with that slight rumble and ticking after reassembling such reels, new or very lightly used reels, which rules out the possibility of wear and tear in the parts ?Which exactly ( change in the ) parameters is the main cause of those changes and how to avoid, or even better, return those in the original state ?

Not completely sure what you are asking. If a reel seems to work after a re-assembly, but just doesn't sound right, it could be several things. Sometimes, having a clip or washer turned the wrong away will do it. Shims left out or need to be re-shimmed, especially on some Mitchell spinning reels. Or, a loose shim left inside. Antireverse not reinstalled properly. Yes, a schematic will help. but on disassembly, you need to watch carefully what is coming off and how, taking pictures, making, notes/drawings, etc.

yes, all said is true,but there is also the case of every item going back the right way and it still doesnt return to the old - new feel..One of the assumptions by some, that I`ve read on this theme is that often and inevitably it is the rotor nut that changes position of the pinnion when screwed back onand that nut needs very careful and always diferent amount of pressure, just because with every unscrewing it is slightly different, rotor - pinnion - nut - shim position even if there was the same exact pressure amount and number of turns made..

I did, magnificent topicand I spreaded it here in serbian forumsit covers a lot of the things I ask here about, but there is more to be figured, hopefully.One thought I just had, is that, graphite bodied reels with use get ever so slightly misshapen in all those pressured areas, it is a fraction of a fraction of a milimeter, but when it happens all over the critical points it eventualy shows, in that "sounding" different way..